Catalyst deactivation due to carbon accumulation is one of the most difficult challenges in the design and preparation of catalysts for the reforming of hydrocarbon fuels. Carbon deposits decrease catalyst activity by blocking active sites, causing attrition of catalyst particles and results in increasing pressure drop and ultimately discontinuation of the process.
Carbon may form readily via, for example, hydrocarbon decomposition and CO disproportionation. On a supported metal catalyst such as Pt or Ni deposited onto a relatively non-oxygen conducting phase such as alumina, carbon accumulates on the support and at a certain level begins to block the active metal sites, deactivating the catalysts rapidly. This tendency has been related to the concentration of acid sites on the support. Additionally, certain catalytically active metals such as Ni dispersed on a support form a filamentous carbon which tends to lift the catalyst from the support rapidly under operating conditions. Reducing the size of the Ni metal particles has been shown to slow down the overall rate of accumulation of the carbon filament. It is therefore generally understood that the support and the active metal play an important role in ensuring catalyst stability. See e.g., Lercher et al., “Design of stable catalysts for methane-carbon dioxide reforming,” 11th International Congress on Catalysis—40th Anniversary, Studies in Surface Science and Catalysis 101 (1996), among others.
It is also understood that carbon accumulation is mitigated by the oxidation of atomic carbon to CO or CO2 prior to the formation of stable carbon networks on the catalyst surface. Detailed oxygen exchange mechanisms between the catalyst and the gaseous stream have been proposed in a series of well-differentiated steps, initiated by dissociative adsorption of oxygen on the active metal sites. It has been suggested that oxygen exchange rate is controlled by the adsorption-desorption of oxygen on the active metal particles, and that the active metal particles serve as descriptive portholes for the subsequent migration of oxygen to the support. The specific action between carbon and oxygen leading to CO or CO2 formation and the mitigation of carbon accumulation beyond this point is less well understood, however in certain catalysts it has been demonstrated that the oxygen required for CO or CO2 formation generates from lattice oxygen within the bulk of the catalyst. For example, in evaluations utilizing a gadolinium doped ceria support, 18O and 16O isotopic exchange studies indicate that partial oxidation of methane using a Rh catalyst initially produced predominantly C18O, indicating that the oxygen required for the CO formation originated from the catalyst. See Salazar-Villapando et al., “Role of lattice oxygen in the partial oxidation of methane over Rh/zirconia-doped ceria. Isotopic studies” International Journal of Hydrogen Energy 35 (2010), which is hereby incorporated by reference in its entirety.
Further, it is understood that certain oxides may directly exchange 18O and 16O with a gaseous stream in the absence of a supported metal. See e.g., Martin et al., “Mobility of Surface Species on Oxides. 1. Isotopic Exchange of 18O2 with 16O2 of SiO2, Al2O3, ZrO2, MgO, CeO2, and CeO2—Al2O3. Activation by Noble metals. Correlation with Oxide Basicity”, J. Phys. Chem. 100 (1996).
Active metal sites have been dispersed on oxygen conducting phases for the mitigation of carbon accumulation. In these catalysts, the active metallic sites are deposited directly onto the oxygen conducting phase, and the loading of the isolated metal on the support is based on avoidance of phenomena stemming from the reactive nature of the metal itself. For example, when unconstrained by a crystal structure, the metallic sites are prone to sintering with adjacent metal sites, reacting with the oxygen conducting phase to form intermetallic compounds, or engaging in other thermodynamically favorable reactions which act to degrade the performance of the catalyst. Typically, when loading has been varied and performance evaluated for these dispersed metal catalysts, the optimum point has been identified as some loading which maximizes active metal content while avoiding the degrading tendencies of the metallic sites themselves—such as sintering, intermetallic formation, or other reactions—in order to provide activity and selectivity with an acceptable rate of carbon deposition. In these situations, the behavior of the metallic sites themselves rather than carbon oxidation through the action of the oxygen conducting phase becomes the limiting point. For example, situations may arise where a given metal loading provides for essentially complete carbon oxidation, but where the response of the metal sites at that particular loading leads to rapid catalyst deactivation. See e.g., Ruckenstein et al., “Carbon Deposition and Catalytic Deactivation during CO2 reforming of CH4 over Co/γ-Al2O3 Catalysts”, Journal of Catalysis 205 (2002).
It would be advantageous to provide a catalyst system where an active catalytic component could be dispersed onto an oxygen conducting phase in a manner that mitigates concerns associated with sintering, intermetallic formation, or other reactions typically identified as the limiting impact on support loading. Providing sufficient stability to the active metal sites under applicable operating conditions would provide additional freedoms in the active metal loading and the relative quantities of the active metal and an oxygen conducting phase, offering general improvement in catalytic performance. It would be additionally advantageous if the relationship between the dispersed active metal sites and the oxygen conducting phase provided for an optimization of conversion, product composition, and carbon deposition in an oxidation process, enhancing the performance of the catalyst system over an expected lifetime. It would further be advantageous if the catalyst system incorporated a mode of operation between active metal sites bound within the structure of a crystal lattice and the oxygen conducting phase based on the oxidation conditions of a specific application, in order to prepare the catalyst system for a performance based on the specific application and end-user preferences.
Accordingly, it is an object of this disclosure to provide a catalyst system having a catalytically active phase dispersed on an oxygen conducting solid, where the catalytically active phase incorporates active metal sites bound within the crystal lattice of a host structure.
Further, it is an object of this disclosure to provide a catalysts system whereby the crystal structure having active metal sites may be dispersed on the oxygen conducting phase in a manner providing for optimum product composition with mitigated carbon deposition.
Further, it is an object of the disclosure to provide a means by which an optimum coverage ratio of the catalytically active phase on the oxygen conducting phase may be determined for a specific application of an oxidation process.
Further, it is an object of the disclosure to provide a means by which oxygen exchange between a gaseous stream containing hydrocarbons and an oxidant may be optimized based on catalysis at the active sites, as opposed to limitations imposed by sintering, intermetallic formation, or other reactions typically identified as the limiting impact on metal loading.
Further, it is an object of the disclosure to provide a means for achieving a mode of operation between active metal sites bound within the structure of a crystal lattice and the oxygen conducting phase based on the oxidation conditions of a specific application, in order to prepare the catalyst system for a performance based on the specific application and end-user preferences.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.